This invention relates to offshore platforms, and specifically to buoyant platforms, such as a tension leg platform (TLP), used to drill into, and/or produce hydrocarbons from, resource formations under a large body of water, such as an ocean, sea, or lake.
TLPs are generally used in offshore locations where the water depth (usually about 1000-1200 feet or higher) precludes the use of rigid structures extending from above the mean water level to the underwater floor foundation. A typical TLP is composed of a hull structure comprised of a horizontal pontoon base and vertical columns, with the hull structure being capable of providing buoyancy to one or more work platforms for drilling or producing activities, the platform(s) being supported in an upper portion of the hull. The hull is generally divided into several ballast compartments, and pumps are provided for removing or adding water thereto.
The TLP is anchored by tendons to pilings in the underwater floor. The TLP is held stationary by tension constraint, i.e., the TLP is deballasted so as to induce constant, vertical buoyant forces on the hull, which are opposed (or constrained) by tension forces in the anchored tendons. Each tendon is considered a xe2x80x9clegxe2x80x9d holding the hull in place, and hence the name: tension leg platform.
A tension leg platform is generally installed by floating the platform-hull structure in a ballast condition to the desired location where the tendons have already been anchored to the seafloor, with the other end freely floating through the aid of detachable buoys or buoyant cans. After the tendons are appropriately attached to the hull, usually by guidelines leading to the production deck or other work platform, the buoys are detached, and the platform-hull structure is rapidly deballasted to float higher in the water and induce the constraining tension forces in the tendons.
Rapid deballasting is crucial during installation to minimize the amount of time that the natural period (or frequency) of the surrounding water equals the natural period of the hull structure. More specifically, in the initial ballast condition, the hull structure has a natural period greater than the natural period of the surrounding water (or wave natural period); conversely, in the final xe2x80x9ctensionedxe2x80x9d position, the hull structure has a natural period less than the wave natural period. Thus, during the deballasting operation, the hull structure will necessarily pass through a time when its natural period and that of the surrounding water are the same. This sets up a potentially unsafe condition, in which the integrity of the entire hull structure is in danger. For this reason, rapid deballasting is essential, so as to minimize time exposure to an unsafe condition.
To provide for rapid deballasting, TLPs have been equipped with one or more pump rooms containing large capacity pumps to draw down the water in the ballast compartments. While this method serves the intended purpose, after the rapid deballasting to install the TLP, the pumps never again need to operate at full capacity, since only ballast trimming operations (to adjust for weight added to, or removed from, the TLP) are needed thereafter. It clearly would be desirable if a rapid deballast system could be provided without the need for one or more large capacity pumps.
The present invention provides a TLP having a simplified ballast system.
The TLP is greatly simplified in providing a simple eductor in one or more drain wells within the hull column(s) to handle ballast trimming operations while rapid deballasting is accomplished via one or more large diameter dump valves on one or more of the upper column ballast compartments. As a result, pump room(s) with all associated equipment, e.g., power outlets and cables, lighting, and sea chests are eliminated.